This invention relates to acoustic well logging in general and more particularly to methods and apparatus for generating and transmitting acoustic waves and transmitting them into a formation, such methods and apparatus being adapted particularly for use in borehole acoustic shear wave logging.
It has long been known in the investigation of sub-surface earth formations traversed by a borehole that measurements or "logs" of acoustic energy introduced into the formation can yield extremely useful information about various formation parameters and characteristics. Accordingly, it has been conventional to introduce a logging sonde into the borehole containing some form of acoustic wave generator and receiver. to direct acoustic energy from the generator into the formation adjacent the borehole elevation of interest, and to thereafter record with the receiver the resultant acoustic waves returning from the formation.
In such acoustic well logging techniques, several interrelated constraints combine to severely complicate the task of improving the design of acoustic wave generators or"sources".
First, the source desirably must be of a relatively small size for several reasons. For example, logging tools typically may only have a nominal outside diameter of 4" or the like. Moreover, although the constraint on vertical size of a source would not appear to be so severe, it is nevertheless desirable to also avoid undue length of a logging tool. This is due to the need to insure smooth passage of the sonde through the borehole (which often may be deviated) and the fact that the increasing complexity of downhole instrumentation puts all space within the logging tool at a premium.
Yet another constraint on the design of acoustic logging sources in some applications was that they must emit acoustic waves of a relatively low frequency and yet often of a high power, as, for example, in the case of direct shear wave logging in soft formations or in cross-borehole compressional wave logging.
Referring to shear wave logging in particular as but one example, measurement of the shear wave was often difficult to accomplish due to its relatively small amplitude relative to other "noise" (which may include compressional waves and the like as is well known in the art). Thus, it was frequently desirable to generate strong shear waves within the formation. This, in turn, meant that extremely powerful acoustic wave sources for setting up the shear waves were required, often of a low frequency as, for example, in the case of logging soft formations.
Both requirements of low frequency and high power in acoustic logging sources suggested taking advantage of the relatively larger longitudinal dimension of the sonde in the design of these sources. In particular, necessity to operate sources in the range of 3 KHz or lower with appreciable output power and the resultant lengths of resonating members required to achieve natural frequencies in this range suggested use of the sonde's longitudinal dimension.
However, as aforesaid, not only were source designs hampered by the necessity to restrict source widths (due to the limited diameter of the sonde), but it was further desirable to avoid unduly utilizing space within a sonde in the longitudinal direction.
Accordingly, an acoustic wave source was desired which was small in size (due to practical physical constraints of a logging sonde), yet relatively high in output power and low in its resonant frequency, and further adapted in particular to establish strong acoustic shear waves in the formation sufficient for direct shear wave logging in soft formations.